Expandable mandrel for use in friction stir welding

ABSTRACT

A mandrel that provides a counter-force to the pressure exerted on the outside of a pipe or other arcuate surface by a friction stir welding tool, wherein the mandrel is expandable through the use of a wedge, and wherein the mandrel enables multiple friction stir welding heads to simultaneously perform welding on the arcuate surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to friction stir welding. More specifically, the present invention addresses improvements in the ability to perform friction stir welding of pipe or other arcuate objects, wherein a mandrel is needed to provide a counter-balancing force against the inside of the arcuate surface being welded, to thereby prevent a friction stir welding tool in contact with the outside of the arcuate surface from damaging the workpiece being welded.

2. Description of Related Art

Friction stir welding (hereinafter “FSW”) is a technology that has been developed for welding metals and metal alloys. The FSW process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin or spindle. Force is exerted to urge the spindle and the workpieces together and frictional heating caused by the interaction between the spindle and the workpieces results in plasticization of the material on either side of the joint. The spindle is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing spindle cools to form a weld.

FIG. 1 is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool 10 having a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized workpiece material. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18. The pin 14 is plunged into the workpiece 16 at the joint line 18.

The frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that may be generally indistinguishable from the workpiece material 16 itself, in comparison to other welds.

It is observed that when the shoulder 12 contacts the surface of the workpieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the tool pin 14.

During FSW, the area to be welded and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint. The rotating FSW tool provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the base metal, while transporting metal from the leading face of the pin to its trailing edge. As the weld zone cools, there is typically no solidification as no liquid is created as the tool passes. It is often the case, but not always, that the resulting weld is a defect-free, recrystallized, fine grain microstructure formed in the area of the weld.

Previous patent documents have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, metal matrix composites, ferrous alloys such as steel and stainless steel, and non-ferrous materials. Another class of materials that were also able to take advantage of friction stir welding is the superalloys. Superalloys can be materials having a higher melting temperature bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.

It is noted that titanium is also a desirable material to friction stir weld. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials.

The previous patents teach that a tool is needed that is formed using a material that has a higher melting temperature than the material being friction stir welded. In some embodiments, a superabrasive was used in the tool.

The embodiments of the present invention are generally concerned with these functionally unweldable materials, as well as the superalloys, and are hereinafter referred to as “high melting temperature” materials throughout this document.

Recent advancements in friction stir welding (FSW) technologies have resulted in tools that can be used to join high melting temperature materials such as steel and stainless steel together during the solid state joining processes of friction stir welding.

As explained previously, this technology involves using a special friction stir welding tool. FIG. 2 shows a polycrystalline cubic boron nitride (PCBN) tip 30, a locking collar 32, a thermocouple set screw 34 to prevent movement, and a shank 36. Other designs of this tool are also shown in the prior art of the inventors, and include monolithic tools and other designs.

When this special friction stir welding tool is used, it is effective at friction stir welding of various materials. This tool design is also effective when using a variety of tool tip materials besides PCBN and PCD (polycrystalline diamond). Some of these materials include refractories such as tungsten, rhenium, iridium, titanium, molybdenum, etc.

The inventors have been the leader in developing friction stir welding technology for use with high melting temperature alloys such as steel, stainless steel, nickel base alloys, and many other alloys. This technology often requires the use of a Polycrystalline cubic boron nitride tool, a liquid cooled tool holder, a temperature acquisition system, and the proper equipment to have a controlled friction stir welding process.

Once the technology had been established (current literature indicates the state of the technology) as a superior method for joining these materials, MegaDiamond and Advanced Metal Products (working together as MegaStir Technologies) began searching for applications that would greatly benefit from this technology. One of the largest applications for friction stir welding (FSW) is joining pipe lines. Joining pipe line is extremely costly because of the manpower and equipment needed to weld and move needed components. FIG. 3 shows the manpower and equipment needed to fusion weld a typical pipeline. The pipe 40 is shown with a plurality of welding stations 42 (each of the white enclosures) that are needed to lay down progressive layers of welding wire to create a fusion welded joint between segments of pipe.

Advanced high strength steels (AHSS) are being implemented into pipe lines because less material is needed, higher strength properties are obtained and the total pipeline cost can be lower. The difficulty with AHSS lies in the conventional fusion welding methods being used. It is accepted in the industry that every pipe line joint contains a defect or crack. These defects are accepted because they cannot be eliminated even with sophisticated automated fusion welding systems. Welding AHSS is far more difficult than existing pipe line steels because the material composition inherently causes more fusion welding defects.

FSW has now been established as a viable technology to join pipe segments. A friction stir welding machine 50 to join pipe segments has been developed as shown in FIG. 4. A rotating tool plunges into a joint as it creates frictional heat. Once the tool has plunged into the workpiece cross section, the tool is caused to travel circumferentially around the pipes while the joint is “stirred” together. The FSW tool is then retracted and the machine 50 is moved along the pipe to the next pipe joint to be friction stir welded.

The friction stir welding machine 50 shown in FIG. 4 illustrates the machine that operates on the exterior of the pipe being welded. One of the requirements of FSW in any form is to have a counter-balancing force on the back side (opposite the tool) of the workpiece being joined. This need arises from the large forces that are applied by the tool against the workpiece. The nature of friction stir welding requires that some support be provided to prevent the workpiece from bending or otherwise being damaged. FIG. 5 shows the current design of a rotating mandrel 60 or “pipe pig” that is currently being used when a friction stir welding pipe.

The mandrel 60 is hydraulically actuated to follow the tool path on the inside of the pipe as the tool follows circumferentially around the pipe joint on the exterior. When the pipe joint is complete, the mandrel 60 is reconfigured so that it can be moved to the next pipe joint. While this mandrel 60 is an effective means to provide support on the opposite side of the tool, the hydraulics and controls are expensive and the construction of the pipe is therefore also costly. A mandrel 60 for FSW of a 12 inch pipe diameter using this design also weighs about 800 lb. This means that moving the mandrel requires additional equipment and support. A further disadvantage is that this mandrel configuration must also have additional hydraulics and rams added to align two pipe segments, further adding to the weight of the mandrel 60. While this design is workable in the field, it would be preferable to have a lighter weight and lower cost mandrel design that can add to the speed and reduce the cost of FSW of a pipeline.

Accordingly, what is needed is a less expensive, less complex, and lightweight pipe pig that can be more easily deployed on-site.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an expandable mandrel that is less complex than those used in the prior art.

It is another object to provide an expandable mandrel that is lighter in weight and therefore easier to use than those used in the prior art.

It is another object to provide an expandable mandrel that can easily move along a length of a pipe in order to reposition itself for use in subsequent friction stir welding operations on-site.

The present invention is a mandrel that provides a counter-balancing force to the pressure exerted on the outside of a pipe or other arcuate surface by a friction stir welding tool, wherein the mandrel is expandable through the use of a wedge, and wherein the mandrel enables multiple friction stir welding heads to simultaneously perform welding on the arcuate surface.

These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a tool as taught in the prior art for friction stir welding.

FIG. 2 is a perspective view of a removable polycrystalline cubic boron nitride (PCBN) tip, a locking collar and a shank.

FIG. 3 is a perspective view of a plurality of welding stations that are needed to lay down progressive layers of welding wire to create a fusion welded joint between segments of pipe in the prior art.

FIG. 4 is a perspective view of a friction stir welding machine that is capable of joining pipe segments.

FIG. 5 is a perspective view of a current design of a rotating mandrel “pipe pig” currently being used when friction stir welding pipe.

FIG. 6 is a perspective view of a mandrel shell.

FIG. 7 is a perspective view of a mandrel shell having attached lips for expanding a gap.

FIG. 8 is a perspective view of a mandrel shell showing the means for expanding the gap in the mandrel shell.

FIG. 9 is a cross-sectional perspective view of a mandrel shell and the means for expanding the gap in the mandrel shell.

FIG. 10 is a cross-sectional perspective view of a mandrel shell and the means for expanding the gap in the mandrel shell, disposed inside a pipe.

FIG. 11 is an end view of a mandrel shell showing a system of cable and pins for closing the gap when the expanding wedge is retracted.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the details of the invention in which the various elements of the present invention will be described and discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow.

The presently preferred embodiment of the invention is an expandable mandrel for use in friction stir welding operations on arcuate surfaces such as pipe. An expandable mandrel concept was developed that proved to be simple, light weight, and inexpensive. The construction of the mandrel is shown in the following steps.

FIG. 6 shows a first embodiment of a mandrel shell 72 that forms an outer shell of the mandrel or “pipe pig” 70 of the present invention. The mandrel shell 72 is a hollow cylinder having an opening or gap 74 along the length thereof. The diameter of the mandrel shell 72 is selected so that the mandrel shell will slide inside of the pipe segments (not shown) that are to be welded, when the gap 74 is allowed to close.

FIG. 6 also illustrates relief cuts 76 that are made on the inside diameter 78 of the mandrel shell 72 so that the mandrel shell can spring and flex at the locations of the relief cuts 76. Once the mandrel shell 72 is machined, lips 80 are welded into place on the inside diameter 78 of the mandrel shell 72 immediately adjacent to the gap 74 as shown in FIG. 7.

Once the lips 80 have been welded into place, the mandrel shell 72 is further modified so that the gap 74 is naturally in a closed position when there is no external force being applied to the mandrel shell. This closing of the gap 74 is accomplished by running a fusion weld bead, as is known to those skilled in the art, parallel to the length of the relief cuts 76, and in equiangular positions relative to each other. In other words, enough weld beads are disposed on the inside of the mandrel shell 72 in uniform locations to distort the mandrel shell so that the gap 74 is closed as a result of the residual stresses caused by the solidifying weld beads. Thus, the mandrel shell 72 now springs back to a closed position if the gap 74 is forced apart.

In FIG. 8, the next component of the pipe pig 70 is to provide a mechanism whereby the mandrel shell 72 can be caused to expand and open the gap 74 when needed. Accordingly, an expanding wedge 82 is provided so that it can be inserted between the lips 80 of the mandrel shell 72. Note that the angle of the expanding wedge 82 that makes contact with the lips 80 is constructed to easily allow the expanding wedge to move upwards into the gap 74, and thereby cause the gap to continue to widen as long as the expanding wedge can be pushed against the lips 80.

Expansion of the mandrel shell 72 stops when the expanding wedge 82 makes contact with the inside of a pipe, or when the outside diameter of the mandrel shell 72 can no longer expand outwards against the inside diameter of a pipe.

FIG. 8 also illustrates a platform or plate 84, and a plurality of hydraulic cylinders 86 that are disposed on the plate. The hydraulic cylinders 86 push against the plate 84 and the expandable wedge 82 to cause the expandable wedge to move upwards into the gap 74. It is envisioned that the bottom of the hydraulic cylinders 86 could also be modified so as to fit the inside of the mandrel shell 72. However, as the mandrel shall is designed to expand and contract, the bottom of the hydraulic cylinders 86 would need to be able to compensate for the shift in shape.

It should be noted that a single hydraulic cylinder 86 could be used in place of the plurality of hydraulic cylinders being shown. Furthermore, the length of the mandrel shell 72, the lips 80, the expandable wedge 82, and the plate 84 can all be modified depending upon the required application. Thus, a system that is smaller in length may be useful in applications where the space or length of horizontal sections within a pipe are restricted.

Similarly, the length of the components listed above might be expanded to enable multiple tools to simultaneously be used to perform friction stir welding on a pipe while the pipe is supported by the single pipe pig 70.

FIGS. 9 and 10 show how the expanding wedge 82 is positioned to slide outwards through the lips 80 of the mandrel shell 72 if the gap 74 is large enough to accommodate the expanding wedge when the gap is as wide as it can be. FIG. 9 is a cross-sectional view of the invention that also shows the hydraulic cylinders 86 in cross-section. FIG. 10 is a cross-sectional view that shows all the elements shown in FIG. 9, but with the addition of a pipe 90. This figure also shows a joint 96 that is the seam between the pipes being friction stir welded.

FIG. 10 shows the expanding wedge 82 fully inserted between the lips 80. When hydraulic pressure is removed from the hydraulic cylinders 86, the mandrel shell 72 retracts and the mandrel shell springs closed. The mandrel shell 72 can now be moved to a different location within the pipe 90. The mandrel shell 72 is moved to the next pipe joint where it is expanded. Hydraulic hoses and fittings that lead to the hydraulic cylinders 86 are not shown. However, these hoses and fittings are disposed on an end of the mandrel shell 72 so that they are coupled to the hydraulic cylinders 86.

It is noted that not only does the pipe pig 70 provide the counter-balancing force necessary for friction stir welding of the pipe 90, but it can also function to further align the segments of the pipe 90

It is also noted that the plate 84 that supports the hydraulic cylinders 86 is coupled to the mandrel shell 72 so the expanding wedge 82 can be retracted from the gap 74 instead of lifting the plate.

The following are modifications that can be made to the mandrel shell 72 design above that can enhance the operation of the pipe pig 70. For example, holes can be machined through the mandrel shell 72 so that air can flow through the holes when the mandrel shell is collapsed. This creates an “air bearing” on the bottom of the mandrel shell 72 so that one person can easily slide the pipe pig 70 to the next pipe joint that is to be friction stir welded.

Another aspect of the invention is that quick disconnects can be used on the hydraulic hoses that are coupled to the hydraulic cylinders 86 so that the hoses can be quickly disconnected and reconnected when the pipe pig 70 is re-positioned at a next pipe joint.

In another aspect of the invention, a variety of materials can be used to construct the mandrel shell 72. Spring steel could be used to always maintain the relaxed closed position of the mandrel shell 72. The material must always be in the elastic region and not be easily stress relieved. This way, the mandrel shell 72 will always keep its shape. If the mandrel shell does start to lose its shape and spring outward when in a relaxed position so that the gap 74 is visible, more welding beads can be run along the length of the inside diameter to restore the residual stresses that cause the mandrel shell 72 to close.

Another aspect of the invention is that expanding wedges can be made in different sizes to compensate for different tolerances of pipe segments.

Another aspect of the invention is that coatings (TiN, TiCN, etc.) can be used on an outer surface of the mandrel shell 72 to thereby prevent the pipe joint from diffusion welding to the mandrel shell during friction stir welding.

Another aspect of the invention is that the invention can be used for any diameter pipe.

It is noted that a rod is attached (not shown) that feeds hydraulic hoses through the next section of pipe.

Another aspect of the present invention has to do with a means for pulling the mandrel shell 72 closed when in a relaxed position. As shown in FIG. 11, the expanding wedge 82 can include posts or pins 92 and a cable 94 disposed therebetween. The cable 94 is run around a pin 92 on both lips 80 of the mandrel shell 72. When the expanding wedge 82 is retracted, the cable 94 performs the function of pulling on the two lips 80 so that they are forced to come together and close the gap 74. It is anticipated that this system of pins 92 and cable 94 can be disposed on both ends of the mandrel shell 72 if needed.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements. 

1. A mandrel for use in friction stir welding of a pipe, said mandrel comprising: a mandrel shell formed as a hollow cylinder having an outer diameter that is smaller than an inner diameter of the pipe, wherein the cylinder has a gap therein that is perpendicular to a top edge and a bottom edge; and a means for expanding the mandrel shell such that the gap widens to a desired width.
 2. The mandrel as defined in claim 1 wherein the means for expanding the mandrel shell further comprises: at least one hydraulic actuation device; and an expanding wedge coupled to the at least one hydraulic actuation device and disposed so as to widen the gap when the at least one hydraulic actuation device is actuated.
 3. The mandrel as defined in claim 2 wherein the at least one hydraulic actuation device is a hydraulic cylinder.
 4. The mandrel as defined in claim 2 wherein the at least one hydraulic actuation device further comprises a platform on which the at least one hydraulic actuation device can rest.
 5. The mandrel as defined in claim 4 wherein the platform is further comprised of a plurality of holes disposed therethrough.
 6. The mandrel as defined in claim 2 wherein the mandrel is further comprised of a plurality of relief cuts in the inside diameter of the mandrel, wherein the relief cuts are perpendicular to the top and bottom edges of the mandrel.
 7. The mandrel as defined in claim 2 wherein the mandrel is further comprised of two lips, each lip disposed adjacent to and on either side of the gap.
 8. The mandrel as defined in claim 7 wherein each of the lips is tapered away from each other and the gap.
 9. The mandrel as defined in claim 2 wherein the gap in the mandrel shell is closed when the mandrel shell is at rest.
 10. The mandrel as defined in claim 9 wherein the mandrel shell has residual stresses that cause the mandrel shell to be closed at the gap.
 11. The mandrel as defined in claim 10 wherein the residual stresses are created by disposing a plurality of fusion weld beads parallel to the gap and spaced equidistantly from each other in the mandrel shell.
 12. The mandrel as defined in claim 2 wherein the size of the expanding wedge can be modified so as to cause the mandrel to expand more when a larger expanding wedge is used, and to cause the mandrel shell to expand to a lesser degree when a smaller expanding wedge is used, to thereby accommodate different diameter pipes using the same mandrel shell.
 13. The mandrel as defined in claim 2 wherein an outer diameter of the mandrel shell is coated with a material to prevent a pipe from diffusion welding to the mandrel shell during friction stir welding.
 14. The mandrel as defined in claim 7 wherein the mandrel is further comprised of a means for coupling edges of the gap to the expandable wedge, wherein retracting the expandable wedge from the gap causes the edges of the gap to move toward each other.
 15. The mandrel as defined in claim 7 wherein the mandrel is further comprised of: a first and a second post disposed in first ends of the lips; a third and fourth post disposed in a first end of the expandable wedge; and a cable coupled to the third and fourth posts, and disposed around the first and second posts, wherein retracting the expandable wedge from the gap causes the cable close the gap.
 16. A method for providing a mandrel inside a pipe that is being friction stir welded, said method comprising the steps of: (1) providing a mandrel shell formed as a hollow cylinder having an outer diameter that is smaller than an inner diameter of the pipe, wherein the cylinder has a gap therein that is perpendicular to a top edge and a bottom edge, and providing a means for expanding the mandrel shell such that the gap can be widened to a desired width; and (2) expanding the mandrel shell such that an outer diameter of the mandrel shell is in contact with an inner diameter of the pipe.
 17. The method as defined in claim 16 wherein the method is further comprised of the steps of: (1) providing at least one hydraulic actuation device, and providing an expanding wedge coupled to the at least one hydraulic actuation device; and (2) widening the gap when the at least one hydraulic actuation device is actuated.
 18. The method as defined in claim 17 wherein the method is further comprised of the of the step of creating residual stresses in the mandrel shell that cause the mandrel shell to be closed at the gap when no forces are applied to the mandrel shell.
 19. The method as defined in claim 18 wherein the method is further comprised of the step of retracting the expandable wedge from the gap to thereby cause edges of the gap to move toward each other.
 20. The method as defined in claim 16 wherein the method is further comprised of the step of creating a plurality of cuts in the mandrel shell to enable the mandrel shell to expand and contract. 